Updated cost-effectiveness and risk-benefit analysis of two infant rotavirus vaccination strategies in a high-income, low-endemic setting.

BACKGROUND AND OBJECTIVE: Since 2013, a biennial rotavirus pattern has emerged in the Netherlands with alternating high and low endemic years and a nearly 50% reduction in rotavirus hospitalization rates overall, while infant rotavirus vaccination has remained below 1% throughout. As the rotavirus vaccination cost-effectiveness and risk-benefit ratio in high-income settings is highly influenced by the total rotavirus disease burden, we re-evaluated two infant vaccination strategies, taking into account this recent change in rotavirus epidemiology. METHODS: We used updated rotavirus disease burden estimates derived from (active) surveillance to evaluate (1) a targeted strategy with selective vaccination of infants with medical risk conditions (prematurity, low birth weight, or congenital conditions) and (2) universal vaccination including all infants. In addition, we added herd protection as well as vaccine-induced intussusception risk to our previous cost-effectiveness model. An age- and risk-group structured, discrete-time event, stochastic multi-cohort model of the Dutch pediatric population was used to estimate the costs and effects of each vaccination strategy. RESULTS: The targeted vaccination was cost-saving under all scenarios tested from both the healthcare payer and societal perspective at rotavirus vaccine market prices (€135/child). The cost-effectiveness ratio for universal vaccination was €51,277 at the assumed vaccine price of €75/child, using a societal perspective and 3% discount rates. Universal vaccination became cost-neutral at €32/child. At an assumed vaccine-induced intussusception rate of 1/50,000, an estimated 1707 hospitalizations and 21 fatal rotavirus cases were averted by targeted vaccination per vaccine-induced intussusception case. Applying universal vaccination, an additional 571 hospitalizations and <  1 additional rotavirus death were averted in healthy children per vaccine-induced intussusception case. CONCLUSION: While universal infant rotavirus vaccination results in the highest reductions in the population burden of rotavirus, targeted vaccination should be considered as a cost-saving alternative with a favorable risk-benefit ratio for high-income settings where universal implementation is unfeasible because of budget restrictions, low rotavirus endemicity, and/or public acceptance.

Quantification of the horizontal transmission of Mycoplasma synoviae in non-vaccinated and MS-H-vaccinated layers.

The number of newly infected birds attributable to one infectious bird per day (= transmission rate ß) was assessed in non-vaccinated and MS-H-vaccinated experimental specified pathogen-free White Leghorns after Mycoplasma synoviae challenge. Furthermore, the effect of vaccination on the shedding of the challenge strain was determined. The following groups were made: a negative control group (n = 5), a vaccinated (MS-H vaccine by eye drop (>105.7 colour changing units/bird)) non-challenged group (n = 5), two non-vaccinated challenged groups (n = 18 each) and two vaccinated challenged groups (n = 18 each). In the challenged groups, six seeder birds were intratracheally inoculated with 105.4 colony forming units (CFUs)/bird. Trachea swabs were taken at day (D)2, D3, D4, D5, D7, D9, D11, D14, D17, D21, D25, D28, D32, D35, D42 and D46 after contact with seeders and analyzed with a quantitative PCR able to detect the vaccine and field strain separately. The transmission rate and shedding were estimated using the susceptible exposed infectious transmission model and a linear mixed model, respectively. The mean shedding of the challenge strain was 106.4 CFU equivalents M. synoviae/g trachea mucus in vaccinates shedding MS-H, while in the birds not shedding the vaccine (non-vaccinates and vaccinates not shedding MS-H) it was 106.9 CFU equivalents M. synoviae/g trachea mucus. In vaccinates shedding MS-H, ß was 0.0012 (95% C.I.: 0.00048 - 0.0024), while in birds not shedding vaccine (non-vaccinates and vaccinates not shedding MS-H) a significantly higher ß of 0.022 (95% C.I.: 0.015 - 0.031) was found.

Quantification of parasite shedding and horizontal transmission parameters in Histomonas meleagridis-infected turkeys determined by real-time quantitative PCR.

To gain more insight into the within flock transmission of Histomonas meleagridis, the shedding of parasites was quantified by a newly developed real-time quantitative (q)PCR and the basic reproduction number (R0) and the mean number of secondary infections per infectious bird per day in a susceptible population (ß) of H. meleagridis in the absence of heterakis were assessed. Forty turkeys were divided into two groups of 10 and 30 birds at 14 days of age. Birds of the first group were inoculated with 200,000 histomonads each, the second group served as a susceptible contact group. Cloacal swabs were taken at -1, 1, 4, 7, 9, 11, 14, 18 and 21 days post inoculation (p.i.) to assess the shedding of the parasite by the qPCR (detection limit 330 histomonads/ml droppings). The experiment ended at 28 days p.i. Mortality was 100% in the inoculated birds and started at day 12 p.i., while in the contacts, it was 83% and started at 16 days p.i. Shedding started 1 day after the inoculation in both groups. The mean shedding levels (and 95% CI) expressed as parasite equivalents per gram cloacal content on a log10 scale in the inoculated, contact birds that died and contact birds alive were 2.0 (1.6-2.4), 1.6 (1.4-1.9) and 1.2 (0.5-2.0), respectively. Birds that died shed histomonas more often and were infectious for 13.4 days; in contrast, those that recovered were infectious for 5.7 days. R0 was estimated to be 8.4 and ß 0.70. Simulations made with the parameters obtained were in agreement with the experimental results, confirming their validity.

The effect of genetic selection for Johne's disease resistance in dairy cattle: Results of a genetic-epidemiological model.

The objective of this study was to model genetic selection for Johne's disease resistance and to study the effect of different selection strategies on the prevalence in the dairy cattle population. In the Netherlands, a certification-and-surveillance program is in use to reduce prevalence and presence of sources of infection in milk by culling ELISA-positive dairy cows in infected herds. To investigate the additional genetic effect of this program, a genetic-epidemiological model was developed to assess the effect of selection of cows that test negative for Johne's disease (dam selection). The genetic effect of selection at the sire level was also considered (sire selection), assuming selection of 80% of sires producing the most resistant offspring based on their breeding values, as well as the combined effect. Parameters assumed to be affected by genetic selection were the length of the latent period, susceptibility (i.e., the number of infectious doses needed to become infected), or the length of susceptible period as a calf. The effect of selection was measured by the time in years required to eliminate infection. Sensitivity analysis was performed for heritability, accuracy of selection, and intensity of selection. For dam selection, responses to selection were small, requiring 379 to 702 yr for elimination. For sire selection, responses were much larger, although elimination still required 147 to 223 yr. The response to selection was largest if genetic selection affected the length of the susceptible period, followed by the susceptibility, and finally the length of the latent period. Genetic selection for Johne's disease resistance by certification and surveillance is too slow for practical purpose, but that selection on the sire level is able to contribute to the control of Johne's disease in the long run.

Eimeria acervulina: the influence of inoculation dose on transmission between broiler chickens.

The course and clinical appearance of an Eimeria species infection in chicken flocks depend on the response of an individual bird to infection and on population-dynamics of the infection in the flock. Differences in ingested numbers of oocysts may affect oocyst load in the flock and the subsequent infectious dose for not yet infected birds. To study the link between numbers of oocysts excreted by infected birds and transmission of Eimeria acervulina, experiments were carried out with 42 pairs of broiler chickens using inoculation doses with 5, 50, 500 or 50,000 sporulated oocysts. In each pair one bird was inoculated and the other bird was contact-exposed. All contact birds became infected, which occurred on average within 34h after exposure to an inoculated bird. Although a higher inoculation dose resulted in higher oocyst excretion in inoculated and contact-infected birds, only small non-significant differences in transmission rates between groups were found.

[No scientific lower threshold for compulsory vaccination]. / Geen wetenschappelijke ondergrens voor verplicht vaccineren.

The national vaccination rate in young children in the Netherlands has decreased in recent years. This has led to social and political discussions, for instance about compulsory vaccination for children in child-care. The national commission on child-care and vaccination has advised that vaccination should be made compulsory when the rate of vaccination has declined to a pre-determined lower threshold, to be determined by the government. A frequently quoted lower threshold is 95%. The idea behind this is the concept of a critical vaccination rate, a threshold needed for elimination of an infection in a large, well-mixed population. In this article we argue why the critical vaccination rate does not offer a scientific basis for a lower threshold to the national vaccination rate.

Quantification of horizontal transmission of Salmonella enterica serovar Enteritidis bacteria in pair-housed groups of laying hens.

An important source of human salmonellosis is the consumption of table eggs contaminated with Salmonella enterica serovar Enteritidis. Optimization of the various surveillance programs currently implemented to reduce human exposure requires knowledge of the dynamics of S. Enteritidis infection within flocks. The aim of this study was to provide parameter estimates for a transmission model of S. Enteritidis in laying-type chicken flocks. An experiment was carried out with 60 pairs of laying hens. Per pair, one hen was inoculated with S. Enteritidis and the other was contact exposed. After inoculation, cloacal swab samples from all hens were collected over 18 days and tested for the presence of S. Enteritidis. On the basis of this test, it was determined if and when each contact-exposed hen became colonized. A transmission model including a latency period of 1 day and a slowly declining infectivity level was fitted. The mean initial transmission rate was estimated to be 0.47 (95% confidence interval [CI], 0.30 to 0.72) per day. The reproduction number R(0), the average number of hens infected by one colonized hen in a susceptible population, was estimated to be 2.8 (95% CI, 1.9 to 4.2). The generation time, the average time between colonization of a "primary" hen and colonization of contact-exposed hens, was estimated to be 7.0 days (95% CI, 5.0 to 11.6 days). Simulations using these parameters showed that a flock of 20,000 hens would reach a maximum colonization level of 92% within 80 days after colonization of the first hen. These results can be used, for example, to evaluate the effectiveness of control and surveillance programs and to optimize these programs in a cost-benefit analysis.

Assessing HIV care and unmet need: eight data bases and a bit of perseverance.

This study integrated healthcare information from multiple data sources to measure access to HIV primary care in the St. Louis, Missouri area between 1998-2002. We describe the process of creating the collective database and the degree to which each dataset contributed to the calculation of global variables such as evidence of HIV primary care. Descriptive analyses were used to measure evidence of HIV primary among the included data sources. This study was the first of its kind to study HIV primary healthcare access over a period of five years with integrated databases. Findings reinforce the importance of HIV laboratory values as indicators of access to HIV primary healthcare, particularly in the absence of other health data sets. Limitations to the study were posed by data availability and integration of data sources with varying purposes and sophistication.

A simple model for the within-host dynamics of a protozoan parasite.

The dynamics of parasite-host systems can be complicated if the parasite life cycle contains an obligatory environmental stage and if the hosts' immunity increases upon re-infection. The dynamics then greatly depend on the relation between infection history and parasite uptake and excretion of individual hosts. In an effort to better understand such systems, we study Eimeria spp. in chickens as our model. In this paper we take a first step and study the within-host dynamics of Eimeria spp. transmitted through oocysts in the environment, with a mathematical model for the parasite life cycle in discrete time, interacting with a single variable describing the immune response. The model can explain various types of oocyst input-output behaviour as described in previous experiments, in particular the characteristic crowding effect, which causes a decreasing oocyst production with increasing single dose oocyst uptake. Oocyst excretion during constant oocyst uptake (trickle infection) and the immunizing effect of single and trickle infections also appears in accordance with published experiments. The model seems a good description of oocyst input-output behaviour in individual hosts; it provides a solid basis for the study of between-host dynamics, where individuals interact in a common environment, thereby affecting their own and each other's infection pattern.

The effectiveness of classical swine fever surveillance programmes in The Netherlands.

Consequences of classical swine fever (CSF) epidemics depend on the control measures, but also on the number of infected herds at the end of the high-risk period (HRP). Surveillance programmes aim to keep this number as low as possible, so the effectiveness of surveillance programmes can be measured by the number of infected herds at the end of the HRP. In this paper, an evaluation of the effectiveness of the following five Dutch CSF surveillance programmes is presented: (1) routine gross pathology of severely diseased pigs; (2) routine virological tests of tonsils of all pigs, submitted under 1; (3) daily clinical observation by the farmer; (4) periodic clinical inspection by a veterinarian; (5) leucocyte counts in blood samples from diseased animals on a herd where antimicrobial 'group therapy' is started. The evaluation was done by a modelling study, in which virus transmission, disease development, and actions and diagnostic tests in surveillance programmes were simulated. Also, the yearly costs of the programmes were calculated, and direct costs of CSF epidemics were related to the number of infected herds at the end of the HRP. It appeared that the current Dutch surveillance programmes, without the leucocyte counts, keep the number of infected herds at the end of the HRP below 20 with 95% probability. Leaving out the most-expensive programme of periodic inspection (12.5M per year) does not change this result - indicating that (for CSF surveillance) the programme could well be stopped. If the leucocyte programme, which is currently not effective due to the low sample submission rate, optimally were applied, the 95th percentile could be reduced to 10 infected herds. However, whether application is beneficial is unclear, because of uncertainty of the economic benefits due to the many expected false-positive herds each year.

A model of molecular circadian clocks: multiple mechanisms for phase shifting and a requirement for strong nonlinear interactions.

A fundamental question in the field of circadian rhythms concerns the biochemical and molecular nature of the oscillator. There is strong evidence that circadian oscillators are cell autonomous and rely on periodic gene expression. In Drosophila, Neurospora, Aplysia, and vertebrates, circadian oscillators are thought to be based on molecular autoregulatory loops composed of transcription, translation, and negative feedback by proteins on nuclear transcription. By studying a mathematical model of molecular clocks based on this general concept, the authors sought to determine which features such clocks must have to generate robust and stable oscillations and to allow entrainment by external stimuli such as light. The model produced circadian oscillations as an emergent property even though a time delay in protein synthesis and rate constants of the feedback loop were much shorter than 24 h. Along with the delay in protein production, strong nonlinear interactions in macromolecular synthesis and nuclear feedback appeared to be required for the model to show well-behaved oscillatory behavior. Realistic phase-shifting patterns induced by external stimuli could be achieved by multiple mechanisms-namely, up- and downward perturbations of protein or mRNA synthesis or degradation rates. The model makes testable predictions about interactions between clock elements and mechanisms of entrainment and may help to understand the functions of the intricate molecular interactions governing circadian rhythmogenesis.

Eight challenges in modelling infectious livestock diseases.

The transmission of infectious diseases of livestock does not differ in principle from disease transmission in any other animals, apart from that the aim of control is ultimately economic, with the influence of social, political and welfare constraints often poorly defined. Modelling of livestock diseases suffers simultaneously from a wealth and a lack of data. On the one hand, the ability to conduct transmission experiments, detailed within-host studies and track individual animals between geocoded locations make livestock diseases a particularly rich potential source of realistic data for illuminating biological mechanisms of transmission and conducting explicit analyses of contact networks. On the other hand, scarcity of funding, as compared to human diseases, often results in incomplete and partial data for many livestock diseases and regions of the world. In this overview of challenges in livestock disease modelling, we highlight eight areas unique to livestock that, if addressed, would mark major progress in the area.

Classical swine fever (CSF) marker vaccine. Trial II. Challenge study in pregnant sows.

The efficacy of two marker vaccines against classical swine fever (CSF) was tested in a large scale laboratory trial in several National Swine Fever Laboratories (NSFL) of the EU member states. The vaccines were: BAYOVAC CSF Marker (Vaccine A) from Bayer, Leverkusen, Germany and PORCILIS PESTI (Vaccine B) from Intervet, Boxmeer, The Netherlands. At the NSFL of Belgium, The Netherlands and Germany experiments were carried out to examine the ability of the vaccines to prevent transplacental transmission of CSF virus. In Belgium and The Netherlands pregnant sows were vaccinated once and challenged with virulent CSF virus 14 days later, which was around day 60 of gestation. At the NSFL in Germany sows were vaccinated twice, on days 25 and 46 of pregnancy and were challenged fourteen days after booster vaccination (day 60 of gestation). Apart from minor inflammatory reactions in some sows, no reactions post vaccination were noticed in either vaccine group. Sows vaccinated with Vaccine A were better protected against clinical CSF than sows vaccinated with Vaccine B. The antibody response after vaccination with Vaccine A was more pronounced than after vaccination with Vaccine B. After single vaccination six out of eight sows vaccinated with Vaccine A and all eight sows vaccinated with Vaccine B had viraemic piglets. After double vaccination one out of four litters from sows vaccinated with Vaccine A and four out of five litters from sows vaccinated with Vaccine B were found to be viraemic. However, both vaccines reduced the transmission probability significantly (Vaccine A: P=0.004, Vaccine B: P=0.024) after booster vaccination. However, Vaccine A appeared in this regard more potent as the estimated probability of fetal infections was lower. Nevertheless the risk of virus spreading after vaccination via transplacental transmission is still present and has to be addressed from an epidemiological point of view.

Quantification of the effect of control strategies on classical swine fever epidemics.

Emergency vaccination during an epidemic of classical swine fever virus (CSFV) has become a serious option because of the ethical problems of strategies with massive culling and the availability of a marker vaccine that reduces virus transmission. Here we present a model of between-herd CSFV transmission, which quantifies the effect of control strategies with and without vaccination. We estimate the model parameters from data of the Dutch CSFV epidemic of 1997/1998. With the model, a set of control strategies is compared, consisting of five control measures in several combinations. Consequently, the following general requirements of successful strategies can be formulated. First, to achieve extinction of a CSFV epidemic, transmission through transport should be prevented and the indirect virus transmission, i.e. all transmission not through animal contacts, should at least be halved, either by vaccination or by culling of the susceptible pig population. Second, to minimize the size and duration of an epidemic, the extinction requirements should be met quickly and indirect virus transmission should be reduced by far more than a half. Although the origin of the model parameters let the requirements in fact be only applicable for the south-eastern part of the Netherlands, it is argued that epidemics in other areas will not need stricter control strategies.

Spatial and stochastic simulation to compare two emergency-vaccination strategies with a marker vaccine in the 1997/1998 Dutch Classical Swine Fever epidemic.

Two alternative emergency-vaccination strategies with a marker vaccine that could have been applied in the 1997/1998 Dutch Classical Swine Fever (CSF) epidemic were evaluated in a modified spatial, temporal and stochastic simulation model: InterCSF. In strategy 1, vaccination would be applied only to overcome a shortage in destruction capacities. Destruction of all pigs on vaccinated farms distinguishes this strategy from strategy 2, which assumes intra-Community trade of vaccinated pig meat. InterCSF simulates the spread of CSF between farms through local spread and three contact types. Disease spread is affected by control measures implemented through different mechanisms. Economic results were generated by a separate model that calculated the direct costs (including the vaccination costs) and consequential losses for farmers and related industries subjected to control measures. The comparison (using epidemiological and economic results) between the different emergency-vaccination strategies with an earlier simulated preventive-slaughter scenario led to some general conclusions on the Dutch CSF epidemic. Both emergency-vaccination strategies were hardly more efficient than the non-vaccination scenario. The intra-Community trade strategy (vaccination-strategy 2) was the least costly of all three scenarios.

Dating scripts of gay men and lesbians.

Scripts for a first date for 51 gay men and 44 lesbians were explored. Well-defined scripts were found for both hypothetical and actual accounts. Hypothetical scripts contained fourteen actions for gay men and lesbians; eleven were common to both. Gay men's actual scripts had eighteen actions and lesbians' had nineteen, with twelve common to both. Gay men's scripts were more sexually oriented and less intimacy-focused than lesbians'. However, scripts for both genders were free of many other aspects of traditional heterosexual roles and involved some actions unique to this population.

Simulation study of the mechanisms underlying outbreaks of clinical disease caused by Actinobacillus pleuropneumoniae in finishing pigs.

Actinobacillus pleuropneumoniae is a major cause of respiratory disease in pigs. Many farms are endemically infected without apparent disease, but occasionally severe outbreaks of pleuropneumonia occur. To prevent and control these outbreaks without antibiotics, the underlying mechanisms of these outbreaks need to be understood. Outbreaks are probably initiated by a trigger (common risk factor) changing the host-pathogen interaction, but it is unclear whether this trigger causes all cases directly (trigger mechanism), or whether the first case starts a transmission chain inducing disease in the infected contacts (transmission mechanism). The aim of this study was to identify conditions under which these mechanisms could cause A. pleuropneumoniae outbreaks, and to assess means for prevention and control. Outbreaks were first characterised by data from a literature review, defining an average outbreak at 12 weeks of age, affecting 50% of animals within 4 days. Simple mathematical models describing the two mechanisms can reproduce average outbreaks, with two observations supporting the trigger mechanism: (1) disease should be transmitted 50 times faster than supported by literature if there is a transmission chain; and (2) the trigger mechanism is consistent with the absence of reported outbreaks in young pigs as they have not yet been colonised by the bacterium. In conclusion, outbreaks of A. pleuropneumoniae on endemic farms are most likely caused by a trigger inducing pneumonia in already infected pigs, but more evidence is needed to identify optimum preventive interventions.

A cohort study on Actinobacillus pleuropneumoniae colonisation in suckling piglets.

Actinobacillus pleuropneumoniae causes respiratory disease in pigs and despite the use of preventive measures such as vaccination and antimicrobials clinical outbreaks still occur. At weaning often many piglets are not colonised. If differences in prevalence between litters are large and if factors were known that could explain these differences, this may provide an opportunity to raise groups of A. pleuropneumoniae free piglets. To this end, a cohort study was performed on two endemically infected farrow-to-finish farms. Seventy-six of 133 sows were selected using stratified random selection by parity. Farmers complied with a strict hygiene and animal management protocol to prevent transmission between litters. Tonsil brush and serum samples taken three weeks before parturition were tested for antigen with an apxIVA qPCR and antibodies with Apx and Omp ELISAs, respectively. Three days before weaning tonsil brush samples from all piglets (n=871) were collected and tested for antigen. Whereas all sows tested positive both in serology tests as well as qPCR, 0.41 of the litters tested fully negative and 0.73 of all piglets tested negative. The proportion of positively tested piglets in positive litters ranged from 0.08-1.0 (median=0.36). A grouped logistic regression model with a beta binomial distribution of the probability for piglets to become infected was fitted to the data and associations with explanatory variables were explored. To test the possibility that alternatively the clustering was caused by onwards transmission among the piglets, a transmission model was fitted to the data incorporating sow-piglet and piglet-piglet transmission, but this model did not fit better. The results of this study showed that the number of colonised suckling piglets was highly clustered and mainly attributable to the variability of infectiousness of the dam, but no dam related risk factor for colonisation status of litter or piglets within litters could be identified.

Transmission of Actinobacillus pleuropneumoniae among weaned piglets on endemically infected farms.

Clinical outbreaks due to Actinobacillus pleuropneumoniae occur recurrently, despite the wide-scale use of antimicrobials or vaccination. Therefore, new approaches for the prevention and control of these outbreaks are necessary. For the development of alternative measures, more insight into the transmission of the bacterium on farms is necessary. The aim of this cohort study was to quantify transmission of A. pleuropneumoniae amongst weaned piglets on farms. We investigated three possible transmission routes: (i) indirect transmission by infected piglets within the same compartment, (ii) transmission by infected pigs in adjacent pens and (iii) transmission by direct contact within pens. Additionally, we evaluated the effect of independent litter characteristics on the probability of infection. Two farms participated in our study. Serum and tonsil brush samples were collected from sows pre-farrowing. Serum was analysed for antibodies against Apx toxins and Omp. Subsequently, tonsil brush samples were collected from all piglets from these dams (N=542) in three cohorts, 3 days before weaning and 6 weeks later. Tonsil samples were analysed by qPCR for the presence of the apxIVA gene of A. pleuropneumoniae. Before weaning, 25% of the piglets tested positive; 6 weeks later 47% tested positive. Regression and stochastic transmission models were used to assess the contribution of each of the three transmission routes and to estimate transmission rates. Transmission between piglets in adjacent pens did not differ significantly from that between non-adjacent pens. The transmission rate across pens was estimated to be 0.0058 day(-1) (95% CI: 0.0030-0.010), whereas the transmission rate within pens was ten times higher 0.059 day(-1) (95% CI: 0.048-0.072). Subsequently, the effects of parity and serological response of the dam and litter age at weaning on the probability of infection of pigs were evaluated by including these into the regression model. A higher dam ApxII antibody level was associated with a lower probability of infection of the pig after weaning; age at weaning was associated with a higher probability of infection of the pig after weaning. Finally, transmission rate estimates were used in a scenario study in which the litters within a compartment were mixed across pens at weaning instead of raising litter mates together in a pen. The results showed that the proportion of infected piglets increased to 69% if litters were mixed at weaning, indicating that farm management measures may affect spread of A. pleuropneumoniae.

Transmission rate of African swine fever virus under experimental conditions.

African swine fever (ASF) is a highly lethal, viral disease of swine. No vaccine is available, so controlling an ASF outbreak is highly dependent on zoosanitary measures, such as stamping out infected herds and quarantining of affected areas. Information on ASF transmission parameters could allow for more efficient application of outbreak control measures. Three transmission experiments were carried out to estimate the transmission parameters of two ASF virus isolates: Malta'78 (in two doses) and Netherlands'86. Different criteria were used for onset of infectiousness of infected pigs and moment of infection of contact pigs. The transmission rate (ß), estimated by a Generalized Linear Model, ranged from 0.45 to 3.63 per day. For the infectious period, a minimum as well as a maximum infectious period was determined, to account for uncertainties regarding infectiousness of persistently infected pigs. While the minimum infectious period ranged from 6 to 7 days, the average maximum infectious period ranged from approximately 20 to nearly 40 days. Estimates of the reproduction ratio (R) for the first generation of transmission ranged from 4.9 to 24.2 for the minimum infectious period and from 9.8 to 66.3 for the maximum infectious period, depending on the isolate. A first approximation of the basic reproduction ratio (R0) resulted in an estimate of 18.0 (6.90-46.9) for the Malta'78 isolate. This is the first R0 estimate of an ASFV isolate under experimental conditions. The estimates of the transmission parameters provide a quantitative insight into ASFV epidemiology and can be used for the design and evaluation of more efficient control measures.